I will post this here and also at the "Would it be unreasonable...." thread.
A fast walk with ears forward is a great thing going for you. When you feel that she is working herself up to change gait to the trot lift her head to hollow out her back to make it much harder to go into the trot. If you feel her ease up then drop you reins, figuratively speaking, and continue walking. If she does go into the trot lower her head, apply “light”, (the pressure you would get if you were holing your rein with your pinky finger,) pressure at first then continually and increasingly apply more steady pressure until the instant you feel her starting to drop back down to the walk then release all pressure. Figuratively speaking again, let go of your reins. I say “easy” as the first warning, light pressure as the second warning, and then down comes the hammer. Do this over and over and over. Once you get this going for you do it on the way home. It is a lot of work but eventually all you will have to do is lift your rein make a little contact and she will back down. When this is working for you at the walk work the same deal when she surges at the trot dropping pressure the instant she starts to slow down.
The cavalry stop is great for slowing and stopping a horse in a straight line.
I like to use a Tom Thumb bit with a curb strap. It gives the lift single rein action of a snaffle and also a two rein leveraged pull back if needed.